Under the promotion of the increasingly diverse social demands, the mobile communication technology has developed from analog communication system to the current widely used second generation mobile communication system and 2.5 generation mobile communication system, and until the third generation mobile communication system which will be employed in a wide range in the near future. The development of mobile communication technology not only meets the demands of people on voice service, but also effectively meets the demands for the value-added services including the video, audio and interaction of real-time multimedia services, thereby the mobile communication system plays a more and more important role in people's life and work. Among the above, the trunking communication has increasingly become an indispensable service, the digital trunking system is an important branch of the mobile communication system, and is a special mobile communication system for multiple users sharing and dynamically utilizing a group of wireless channels.
The method of realizing trunking services in CDMA2000 system includes two types: the group call and the private call, which are important service features of the trunking system. The group call means that a single user can talk with all the other users in the group simultaneously in a permitted time. When one user is talking, all the other users in the group must listen to him. Once a user in the group pushes the PPT (Push To Talk) button, other users in the group whose priority level is same with that of the user pushing the PPT button can not break in but only wait till the talk is finished. The private call is one-to-one call between users, a user can talk to the one who he is intended to talk in the group without being heard by other users in the group. The trunking system based on CDMA standard adopts grouping data channels for trunking call service, i.e. to realize fast call setup by adopting Ethernet transmission, rather than the traditional circuit exchange in circuit domain. In order to meet the needs of trunking group call and private call, the trunking system based on CDMA standard adopts the method of fast call access. The features of the trucking communication system based on CDMA standard lie in that it adopts semiduplex communication mode, can realize a fast access and supports the group call, i.e. the calling mode of one-to-multi-users. In trunking communication, the users in the group are required to share the forward channel within a region. When users in the group are to call or talk, they only need to press PTT button, then they are able to talk with other users in the group. This type of PTT service is widely applied to intercommunication system and trunking system, providing command and dispatch functions for group users, such as police, fire-fighting, transportation and communication, air station and aviation, construction site, large-scale gathering, and military dispatching etc. With advancement of society, increase of population mobility, and fast development of mobile communication technology, the range of personal communication is becoming wider and wider. The application of trunking service also gradually turns to the individuals, for example, it may be applied in the chat among fashionable young peoples or small communities, and communications between family members. The trunking communication system based on CDMA standard can attract not only new individual users, but also some professional users for the operators.
The most important character of the trunking communication is that it requires short accessing time and quick acquisition of channel. But the traditional CDMA system adopts the method of serial acquisition. The base station subsystem sends out the Extended Channel Assignment Message (ECAM), then sends empty frames to terminal user on the service channel assigned; the terminal user processes the ECAM message, then turns to the forward service channel, and transmits the prefix-frame on the reverse channel after receiving at least two continuous valid empty frames; after acquiring the reverse channel, the base station subsystem sends Base Station Acknowledge Order (BSAckOrder) message to the terminal user, and points out in the Base Station Acknowledge Order message that the message needs to be verified; the terminal user responses by sending message of Terminal Acknowledge Order (MSACKOrder), and points out whether the message needs to be verified by the base station. The above procedures are on the premise that the terminal acquires the forward channel, if the terminal does not acquire the forward channel, it will not send the reverse prefix-frame. However, this type of serial procedure takes a long time for acquisition, and the signaling handshake can only be proceed after the whole acquisition process is finished. Moreover, the requirement of fast call setup in group call can not be satisfied due to the added time for establishing call. In addition, since trunking communication is mainly applied to emergent dispatch, such as the marine emergency, military dispatching, public security command and rush-to-danger dispatching, which are urgent and have high requirement on time, the call establishing time by the traditional cellular mobile communication system can only meet the requirements of common users, but not the trunking users' requirements on fast response. In order to meet the fast response requirement of the trunking communication, the ordinary call establishing procedure must be reformed to reduce the acquisition time, thereby meeting the fast call setup requirement of the digital trunking communication system. At present, there is still no effective method of fast call setup in the digital trunking system based on CDMA technology.